In a video tape recorder (referred as to VTR hereinafter) of helical scan type, two heads are used for recording or reproducing video signals. Each of the two heads alternately traces a video tape for recording or reproducing a prescribed unit of the video signal (referred as to head signal hereinafter), e.g., one field of the video signal.
In the reproduction operation, these head signals reproduced by the two heads are combined one after the other, as is well known. Thus, a continuous reproduced video signal comprising the head signals is obtained.
In the reproduced video signal, however, the head signals obtained by the two heads are occasionally combined out of synchronozation. Thus, the reproduced video signal becomes discontinuous. When such a discontinuous video signal is displayed on a video screen, e.g. a CRT (Cathode Ray Tube), a skew occurs at that portion corresponding to the discontinuous video signal.
FIG. 1 is a graph for examplarily illustrating the occurrence of the skew. In FIG. 1, two head signals A and B read by two heads are discontinuously combined. The head signals A and B have a plurality of regular horizontal scanning line signals H, respectively. Each of the regular horizontal scanning line signals H has a predetermined standard horizontal period Th.s. For example, the standard horizontal period Th.s of the regular horizontal scanning line signal H is provided at 64 .mu.s. (64/1,000 sec.) in the NTSC color television system.
An irregular horizontal scanning line signal He at a discontinuous coupling portion, however, has an irregular horizontal period Th.e different from the standard horizontal period Th.s. For example, the period Th.e of the irregular scanning line signal He becomes shorter than the standard horizontal period Th.s by a period Te, as shown in FIG. 1. The period Te represents the discrepancy of the irregular horizontal period Th.e from the standard horizontal period Th.s. In some cases the period Th.e of the irregular horizontal scanning line signal He at the discontinuous coupling portion becomes longer than the standard horizontal period Th.s, but this case is not shown in the drawing.
When the irregular horizontal scanning line signal He with a shortened or a lengthened period is displayed, the image corresponding to the discontinuous coupling portion warps. This phenomenon is known as the skew.
Generally, the head signals A and B reproduced by the two heads are combined during a vertical blanking period of the video signal. Thus, the skew does not occur on a visible area of the display at a normal speed reproduction. However, the skew occurs on the visible area of the display at special speed reproductions such as a high speed reproduction. This visible skew at special speed reproductions deteriorates the video image.
Generally, VTRs are provided with a suitable means for preventing the skew at the special speeds. FIG. 2 is a block diagram showing a conventional VTR time base correction circuit for reproduced video signals.
In FIG. 2, an input terminal 10 is provided for receiving a composite video signal Sa. The composite video signal Sa is supplied from a switcher (not shown) which combines two head signals A and B reproduced by two heads (not shown). The composite video signal Sa is applied to an output terminal 12 through a variable delay circuit 14. The variable delay circuit 14 comprises a delay device, such as a charge coupled device (referred as to CCD hereinafter) 16 and a first voltage controlled oscillator (referred as to first VCO hereinafter) 18. The first VCO 18 generates a delay control pulse Sb in response to an error signal Sc applied thereto. The error signal Sc will be described in detail later. The CCD 16 delays the composite video signal Sa by a delay time controlled in response to the frequency of the delay control pulse Sb.
The composite video signal Sa from the input terminal 10 is also applied to a sync signal separator 20. The sync signal separator 20 separates a composite sync signal Sd. The composite sync signal Sd from the sync signal separator 20 is applied to a horizontal sync separator 22. The horizontal sync separator 22 further separates a horizontal sync signal Se. The period, i.e., a so-called horizontal period Th of a specific one of the horizontal sync signals Se varies when a discontinuous coupling of the reproduced video signals occurs.
The horizontal sync signals Se are applied to an error detection circuit 24. The error detection circuit 24 has a typical phase looked loop (referred as to PLL hereinafter) configuration. That is, the error detection circuit 24 comprises a second VCO 26, a phase comparator 28 and a low pass filter (referred as to LPF hereinafter) 30. The second VCO 26 generates a reference signal Sf for a phase comparison. The frequency Ff of the reference signal Sf is controlled by a feedback signal Sg from the LPF 30. The feedback signal Sg will be described later.
The horizontal sync signals Se are applied to a first input terminal 28a of the comparator 28. On the other hand, the reference signal Sf from the second VCO 26 is applied to a second input terminal 28b of the comparator 28. The comparator 28 compares in frequency the horizontal sync signals Se with the reference signal Sf. Thus, the above-mentioned error signal Sc is output from an output terminal 28c of the comparator 28 when the specific horizontal sync signal corresponding to the discontinuous coupling of the reproduced video signals is applied to the comparator 28.
The error signal Sc is not only applied to the variable delay circuit 14, but also applied to the LPF 30. The LPF 30 outputs a DC component of the error signal Sc. The DC component of the error signal Sc corresponds to the above-mentioned feedback signal Sg. The frequency Ff of the reference signal Sf generated by the second VCO 26 is controlled by the feedback signal Sg, i.e., the error signal Sc. The PLL configuration of the error detection circuit 24 operates to lock and stabilize the frequency Ff of the reference signal Sf into an average frequency of the horizontal sync signal Se from the horizontal sync separator 22.
The horizontal sync signal Se is obtained from the composite video signal Ba. In the composite video signal Sa, the regular horizontal scanning line signal H having the standard horizontal period Th.s is far larger in number than the signals having an irregular horizontal period Th.e, e.g., Th.s-Te of the irregular horizontal scanning line signal He. Thus, it is assumed that the average frequency of the horizontal sync signal Se matches with a standard horizontal frequency Fh.s corresponding to the standard horizontal period Th.s.
When the irregular horizontal scanning line signal He having the irregular horizontal period Th.e, e.g., Th.s-Te is applied to the input terminal 10, the frequency Fe of the horizontal sync signal Se differs from the standard horizontal frequency Fh.s, i.e., the frequency Ff of the reference signal Sf. The frequency Ff is locked into the average frequency of the horizontal sync signal Se. The comparator outputs the error signal Sc. The error signal Sc has a frequency Fc corresponding to the difference between the frequency Fe of the horizontal sync signal Se and the frequency Ff of the reference signal Sf, i.e., the standard horizontal frequency Fh.s.
The error signal Sc output by the error detection circuit 24 is applied to the first VCO 18 of the variable delay circuit 14. The first VCO 18 generates the delay control signal Sb in response to the error signal Sc from the error detection circuit 24. The delay control signal Sb has a frequency Fb corresponding to the error signal Sc. The delay control signal Sb is applied to the CCD 16. The CCD 16 delays the composite video signal Sa by a delay time defined by the frequency Fb of the delay control signal Sb.
When an irregular horizontal scanning line signal He is introduced into the input terminal 10, the composite video signal Sa is delayed by the delay time. Thus, the irregular horizontal scanning line signal He extends on the time base axis in response to the delay time. As a result, the irregular horizontal period Th.e of the irregular horizontal scanning line signal He is corrected to the standard horizontal period Th.s.
FIG. 3 is a graph showing the frequency response characteristic of the error detection circuit 24. In FIG. 3, a dotted line graph Ra shows the characteristic of the LPF 30. A solid line graph Rb shows a characteristic of the PLL, i.e., the entire error detection circuit 24.
In such a PLL for time base correction circuits, a lag-lead filter is typically used as the LPF 30 for increasing stability of the PLL. The lag-lead filter can independently control the loop gain and damping factor of the PLL. The frequency response characteristic of the lag-lead filter has two singular points, i.e., a pole point Pp and a zero point Pz, as shown in FIG. 3. Frequencies corresponding to the pole point Pp and the zero point Pz will be referred to as a pole-frequency Fp and a zero-frequency Fz hereinafter.
When the composite video signal Sa with the irregular horizontal scanning line signal He appears on the input terminal 10 (see FIG. 2), the error signal Sc with the frequency Fc which is the difference between the frequency Fe of the horizontal sync signal Se and the frequency Ff of the reference signal Sf, i.e., the standard horizontal frequency Fh.s as described above, is applied to the lag-lead filter type LPF 30 from the comparator 28. If the frequency Fc of the error signal Sc is higher than the zero-frequency Fz, the LPF 30 smooths the error signal Sc. Thus, a feedback signal Sg with the DC component of the error signal Sc is output from the LPF 30.
The feedback signal Sg controls the second VCO 26 so that the frequency Ff of the reference signal Sf is locked into the average frequency of the horizontal sync signal Se from the horizontal sync separator 22, as described above. When the frequency Ff of the reference signal Sf is locked into the average frequency of the horizontal sync signal Se, the frequency Fc of the error signal Sc varies by following the fluctuation of the frequency Fe of the horizontal sync signal Se applied to the comparator 28.
Thus, the error signal Sc applied to the first VCO 18 of the variable delay circuit 14 carries an information regarding the discrepancy period Te (Te=Th.s-Th.e). The first VCO 18 generates the delay control signal Sb. Then the delay control signal Sb with a frequency Fb responding to the discrepancy period Te is applied to the CCD 16. The CCD 16 delays the composite video signal Sa so that the discrepancy period Te of the irregular horizontal scanning line signal He in the current composite video signal Sa is removed. As a result, a corrected composite video signal Sa.c is output from the variable delay circuit 14.
One problem with the above-mentioned system is that the frequency Fc of the error signal Sc is lower than the pole-frequency Fp, the second VCO 26 completely follows the fluctuation of the horizontal sync signal Se. In this case, the error signal Sc output from the error detection circuit 24 becomes almost zero. As the error signal of the almost zero varies little, a delay control signal Sb with a prescribed constant frequency is applied to the CCD 16 from the first VCO 18. Thus, the variable delay circuit 14 fails to correct the time base of the composite video signal Sa.
Generally, the frequency discrepancy of the irregular horizontal scanning line signal He is in a relatively low frequency range, from 60 Hz to 300 Hz. To comply with such a low frequency range, the zero-frequency Fz must be set at a frequency under the lowest frequency, e.g., 60 Hz of the frequency range. When the zero-frequency Fz of lag-lead filter type LPFs is set to such a low frequency, the pole-frequency Fp is also set to a relatively low frequency near the zero-frequency. When the zero-frequency Fz and the pole-frequency Fp are set to such low frequencies, the operation of the PLLs becomes unstable. This is another problem with above-mentioned system.
Accordingly, the conventional time base correction circuit, as shown in FIG. 2, is very unstable. That is, the PLL of the error detection circuit 24 of the conventional time base correction circuit cannot maintain the locked state when a relatively large skew has occurred or when the power source of the VTR has been turned ON.
The conventional time base correction circuit, as shown in FIG. 2, has another drawback as described below. The other drawback will be described in reference to FIG. 4. FIG. 4 is a waveform showing the error signal Sc output from the error detection circuit 24, i.e., the comparator 28 in the PLL. The waveform examplarily shows the error signal Sc, when horizontal scanning line signals are alternately lengthened and shortened by a period Te from a standard horizontal period Th.s. Such a discrepancy +Te or -Te of the horizontal period Th occurs at periodic discontinuous coupling portions of two reproduced video signals. The error signal Sc has a positive peak level +V and a negative peak level -V for the irregular horizontal scanning line signals with the lengthened and shortened horizontal periods, i.e., Th.s+Te and Th.s-Te. When the positive and negative peak levels +V and -V are applied to the first VCO 18 in the variable delay circuit 14, the first VCO 18 controls the CCD 16 so as to correct the time base of the composite video signal Sa.
However, the error signal Sc output from the error detection circuit 24 converges into an average level Vo which corresponds to the average frequency of the horizontal sync signal Se or the standard horizontal frequency Fh.s, immediately after the positive and negative peak levels +V and -V. Thus, the first VCO 18 can not maintain the time base correction for the entire composite video signal Sa.